An analog to digital converter (ADC) receives an analog input signal and converts it into a digital signal. The conversion involves quantization of the analog input signal which maps the almost infinitely variable amplitude of the analog signal to one of a finite series of discrete levels. This many to one mapping invariably introduces error into the digital signal. The difference between the input analog signal and the output digital signal is referred to as the quantization error or noise.
To minimise the quantization noise, automatic gain control (AGC) is often used to adjust the amplitude of the analog signal before it is input to the ADC.
Typically an AGC circuit is configured to calculate the average amplitude of the output of the ADC over a predetermined number of samples (e.g. 10 or 100 samples) and compare this to a target average amplitude. The AGC circuit then adjusts the gain applied to the input signal so that the average amplitude of the ADC output will be equal to the target average amplitude.
However, it is difficult to select a single target average amplitude that will work for a variety of input signals.
The embodiments described below are not limited to implementations which solve any or all of the disadvantages of known systems.